A compression refrigerator comprises a compressor, a condenser, an expansion mechanism (for example, an expansion valve or a capillary tube), an evaporator and the like, and is an apparatus where a cooling is performed using a characteristic of a refrigerant having a high volatility to take away an heat of evaporation from its surroundings upon the evaporation. This apparatus is being used for vending machines such as a cooler, a freezer, an air conditioning equipment, a show case, soft drink, ice cream or the like. In the air conditioning equipment or the vending machine, the compression refrigerator is being used also for performing the heating or keeping a drink or a food under heating, utilizing a heat generated upon the condensation.
As the above-described refrigerant, a chlorine-containing fluorohydrocarbon (CFC or HCFC) is conventionally used, and it has been replace by a chlorine-free fluorohydrocarbon (HFC) in recent years. However, these fluorohydrocarbons have a problem that the former destroys the ozone layer and the latter has a high global warming effect and therefore, in place of these, such a refrigerant that exerts no adverse effect on the environment is being demanded from the viewpoint of global environment protection. Accordingly, a refrigerant such as hydrocarbon or ammonia having a low molecular weight has been reconsidered as an environment-friendly refrigerant because it does not destroy the ozone layer and is extremely low in global warming effect as compared with the fluorohydrocarbon. In particular, ammonia has a high performance coefficient.
As a base oil of a lubricant for refrigerator using an ammonia refrigerant, a mineral oil or alkylbenzene has been conventionally used. However, a polyether compound having compatibility with ammonia has been proposed in recent years. For example, European Patent Publication 490810 discloses a lubricant comprising a polyalkylene glycol which is a copolymer of ethylene oxide and propylene oxide and has an EO/PO ratio of 4/1. European Patent Publication 585934 discloses a lubricant comprising a monofunctional or difunctional polyalkylene glycol having an EO/PO ratio of 2/1 to 1/2. German Patent Publication 4404804 discloses a polyether-base lubricant represented by the general formula:RO—(EO)x—(PO)y—H[wherein R represents C1 to C8 alkyl group, X and Y are numbers from 5 to 55]. International Patent Publication WO 94/12594 discloses a refrigerator oil comprising a polyalkylene glycol diether, which is excellent in the compatibility with ammonia and the stability in an ammoniac atmosphere.
If these polyether compounds having the compatibility with ammonia are used, it is not necessary to provide a lubricant circulator where a lubricant is separated and recovered at the exhaust side of a refrigerating compressor and returned to the compressor.
However, ammonia as a refrigerant is a compound having a high activity, and therefore there is a fear that the ammonia reacts with a base oil or an additive of a lubricant for refrigerator or with the deteriorated material thereof to produce a precipitate (reaction product), which causes blocking of an expansion valve or a capillary portion and the wear of a sliding member, and also accelerate the deterioration of the lubricant. Ammonia has high corrosiveness with respect to a metal material, particularly in the presence of water. Therefore, if ammonia is used together with a polyether compound having high moisture absorption, the corrosion or deterioration of metal materials such as occurrence of rust is accelerated. Further, since an ammonia refrigerant requires a higher pressure driving than a flon-base refrigerant, the refrigerant and the lubricant are exposed to severer conditions such as high temperature and high pressure, and also severer sliding conditions apply. As a result, the increase in the generation of precipitates, the acceleration in the deterioration of a lubricant (base oil and additive) and the increase in the friction or the wear are feared. Accordingly, a lubricant which is stable and high in the lubricity even under the severe conditions encountered when using an ammonia refrigerant, is demanded.
With respect to the polyether compound, polyether compound derivatives (hereinafter referred to as a polyether compound including the derivatives) where the terminal hydroxyl group thereof is substituted by an alkyl group or an acyl group to improve the stability and the moisture absorption, have been proposed. However, this method has disadvantages that a production process for obtaining the derivatives needs to be added and the good compatibility with ammonia deteriorates. On the other hand, the level of the improvement in the stability and the moisture absorption is not fully satisfactory. Thus, the use of a base oil comprising such a polyether compound alone is inadequate for solving the problem mentioned above. Therefore, it is required that a suitable additive is selected and used in the combination of ammonia and a base oil. However, the additive has in many cases higher activity than a base oil and it also easily reacts with ammonia. Therefore, the deterioration of the additive and the generation of precipitates are often induced.
The object of the present invention is to provide a lubricant having excellent compatibility with ammonia and moreover having excellent practical performance such as good stability and lubricity, as a lubricant for a refrigerator using ammonia as the refrigerant.